The invention relates generally to switching power converters such as the DC-to-DC buck converters and the boost or buck-boost converters.
The parallel power units of prior art DC-to-DC converters typically couple their inputs to a common DC voltage source and their outputs to a load, such as a microprocessor. As known in the art, multiple power units replacing a single power unit can sometimes reduce cost by lowering the power and size rating of components. A further benefit is that multiple power units provide smaller per-power-unit peak current levels, combined with smaller passive components.
The prior art also includes switching techniques in parallel-power-unit DC-to-DC converters. By way of example, power units may be switched with pulse width modulation (PWM) or with pulse frequency modulation (PFM). Typically, in a parallel-unit buck converter, the energizing and de-energizing of the inductance in each power unit occurs synchronously with switches coupled to the input, inductor and ground. Additional performance benefit occurs when the switches of one power unit, coupling the inductors to the DC input voltage or to ground, are out of phase with respect to the switches in another power unit. This xe2x80x9cmulti-phase,xe2x80x9d parallel power unit technique results in ripple current cancellation at a capacitor, to which all the inductors are coupled at their output terminals.
It is clear that smaller inductances are needed in DC-to-DC converters to support the response time required in load transients and without prohibitively costly output capacitance. More particularly, the capacitance requirements for systems with fast loads, and large inductors, may make it impossible to provide adequate capacitance configurations, in part due to the parasitic inductance generated by a large physical layout. But smaller inductors create other issues, such as the higher frequencies used in bounding the AC peak-to-peak current ripple within each power unit. Higher frequencies and smaller inductances enable shrinking of part size and weight. However, higher switching frequencies result in more heat dissipation and possibly lower efficiency. In short, small inductance is good for transient response, but large inductance is good for AC current ripple reduction and efficiency.
The prior art has sought to reduce the current ripple in multiphase switching topologies by coupling inductors. For example, one system set forth in U.S. Pat. No. 5,204,809, incorporated herein by reference, couples two inductors in a dual-phase system driven by an H bridge to help reduce ripple current. In one article, Wong, Investigating Coupling Inductors in the Interleaving QSW VRM, IEEE APEC (February 2000), slight benefit is shown in ripple reduction by coupling two windings using presently available magnetic core shapes. However, the benefit from this method is limited in that it only offers slight reduction in ripple at some duty cycles for limited amounts of coupling.
It is, accordingly, an object of the invention to provide a voltage converter, such as a DC-to-DC voltage converter, that reduces or eliminates the afore-mentioned difficulties. One specific object of the invention is to provide a converter with two or more windings wound around a common core to preferentially maximize coupling between windings. These and other objects will be apparent in the description that follows.
In one aspect, a DC-to-DC converter is provided to generate an output voltage from an input voltage. The converter includes first and second inductive windings and a magnetic core. One end of the first winding is switched at about 180 degrees out of phase with one end of the second winding, between ground and the input voltage, to regulate magnitude of the output voltage. Each of the first and second windings is wound about a common core. A pair of windings in proximity to one another or wound about a common core forms a transformer. Of the many common electrical circuit models used to describe a transformer, familiar to those skilled in the art, the xe2x80x9cT-model,xe2x80x9d will be used herein. The T-model comprises two leakage inductances, one associated with each winding, a common magnetizing inductance, and an ideal transformer. The inductance measured with only one of the windings on the core would be the sum of the one winding""s leakage inductance and the magnetizing inductance. The first winding is wound about the core in a first orientation, but the second winding is also wound about the core in the first orientation so as to increase coupling between windings and to reduce ripple current associated with the output voltage. To clarify what is intended by the orientation of the windings, when the two windings both have positive current, the flux generated around the main magnetizing flux path by one should be counterclockwise, whereas the flux generated by the other should be clockwise. When the two windings are wound around opposite sides of a square post, both produce flux in the same direction in Cartesian coordinates, given positive current. The issue of what is meant by the same orientation is discussed further below.
In one preferred aspect, the invention is deployed in the form of a buck converter. Those skilled in the art should appreciate that modifications can be made to form a boost, buck-boost, or other converter, as described herein.
The invention has several advantages in addition to those apparent above. For example, the converter of the invention not only provides ripple cancellation in the output capacitor, but can also provide ripple cancellation in the windings and in the switches. It can do so with two or more windings. Moreover, ripple reduction is minimized with xe2x80x9cperfectxe2x80x9d coupling between the windingsxe2x80x94a feature distinctly absent in the prior art. In a further advantage, the invention operates with a magnetic core shaped in one of multiple geometries, whereas the prior art describes only certain shapes. By way of example, Wong, Investigating Coupling Inductors in the Interleaving QSW VRM, IEEE APEC (February 2000), requires E cores with center legs, and U.S. Pat. No. 5,204,809 discloses a doughnut shaped core. In accord with the invention, the core may take several forms, described below, and additionally can provide more power than an E core of the same physical size, because space for a center leg is not needed. Part of the distinction between the prior art and the present invention can be better understood with reference to the intended use of the inductors. One purpose of integrating two separate inductors in the prior art was to save space on a printed circuit board. Coupling between windings on a common core was, in fact, not desired, except to decrease the number of components and overall component area. The present invention actively seeks to couple windings together on the same core.
The invention also provides methods for magnetically coupling inductive devices in a parallel, multiphase power unit regulator topology to reduce current ripples. The method includes the steps of: orienting, in like direction, first and second windings about a common core to increase coupling between the windings; and alternatively activating one end of the first winding about 180 degrees out of phase with one end of the second winding, between a control voltage and the input voltage, to regulate magnitude of the output voltage, wherein magnetizing inductance substantially equals an inductance of the first winding with the core absent the second winding. In one preferred aspect, the control voltage is ground.
The methods of the invention thus enable the use of smaller inductances for transient response optimization without incurring additional current ripple. The invention accordingly lends itself to scalability in coupled magnetic and multiphase topologies. As the number of phases increases almost arbitrarily, the resulting current ripple will continue to be reduced. Further benefits are achieved in replacing standard separate inductors, one per power unit, with a single combined magnetic structure of the invention, including: size reduction, pick and place assembly time and error reduction, lower design cost, implementation, and ease of manufacturing DC-to-DC converters.
Those skilled in the art should also appreciate that the use of coupled magnetics in accord with the invention can reduce current ripple when switching at a particular frequency. If the original switching frequency is lowered, then magnetic coupling will in fact boost each individual winding""s current ripple frequency back to the original frequency. Accordingly, the switching frequency of the switches will be reduced with the current ripple being the same as with no coupling. This lower frequency switching results in higher system efficiency. In the various implementations of the invention, when same side switches in different power units operate simultaneously, as they might during transients, then the slopes of inductor currents in each power unit will be substantially maximized. Therefore, both steady-state ripple reduction (by minimizing inductor current slew rates) and short-term transient response enhancement (by maximizing inductor current slew rates) can be achieved simultaneously.
The invention also provides a multi-phase DC-to-DC converter for providing an output voltage from an input voltage. The converter has N (Nxe2x89xa72) inductive windings alternatively switched between a control voltage and the input voltage to regulate magnitude of the output voltage. Each of the N windings has a turn-on switching transition separated in switching phase by about 360/N degrees from any other of the windings. Each of the windings also has a turn-off switching transition separated in phase by about 360/N degrees from any other of the windings. Each of the windings is wound on a common magnetic core wherein magnetizing inductance is greater than about three times the leakage inductance of any one of the windings. Each of the N windings is wound about the core in like orientation to increase coupling between windings and to reduce ripple current associated with the output voltage.
In other aspects, the invention provides core structures suitable for multiple windings in a converter such that the multiple windings collectively excite at one excitation pattern, and collectively do not excite at another excitation pattern. By way of example, an oval-shaped core structure and a square core structure are suitable for two windings; however neither are suitable for more than two windings. As another example, a core structure in the shape of a xe2x80x9cladderxe2x80x9dxe2x80x94where one winding is wound around each rung of the ladderxe2x80x94can support two or more windings. Accordingly, by way of specific example, a N-rung ladder structure can support N windings, if desired (Nxe2x89xa72). Another core structure supporting more than two windings, in accord with the invention, includes a wheel-shaped structure with each of the windings wound around separate spokes. Again, N windings may be deployed providing N spokes support the windings. Yet another core structure supporting the invention includes a structure in the shape of two plates (e.g., disks, rectangles, or other shape) coupled by N columns. In each of the aforementioned structures, excitations of the windings can be made to excite all windings, or not. In the case of multiple excitation, for example, by exciting each winding in a ladder configuration, one can make each winding impart flux into neighboring windings in a positive fashion, with flux flow constructively interfering. Alternatively, flux flow from one winding can be made to effectively collide with flux flow of another winding thereby decreasing interaction between windings.
These and other aspects and advantages of the invention are evident in the description which follows and in the accompanying drawings.